This invention relates to a light-emitting device of a spontaneous emission type used, for example, as a display device such as a color display, and also to the use thereof.
In recent years, importance of the interface between the human being and the machine increases, particularly, in multimedia-oriented articles. In order to operate the machine more comfortably and more efficiently, it is necessary to retrieve a satisfactory quantity of information concisely and instantaneously from the machine. To this end, studies have been made on a variety of display devices.
As the scale-down of machines is in progress, there is an increasing demand, day by day, for the display device with respect to its scale-down and thinning. Specific examples of main display devices are described below.
A liquid crystal display that is one of typical display devices is now used as an interface of a variety of articles, and has been widely employed in lap top-type information processing machines and also in living articles including small-sized television sets, watches, desk-top calculators and the like.
While making use of the feature that a liquid crystal is driven at a low voltage with a reduced power consumption, the liquid crystal display has been studied solely as an interface, which is a key of devices including from small-sized to large capacitance display devices.
However, the liquid crystal display is of the light-receiving type, for which back light is essential. For the drive of the back light, it is necessary to provide electric power that is greater than that for a liquid crystal. Accordingly, although a battery is built in, limitation is placed on power supply, with the attendant problem that the working time becomes shortened, thus imposing a limit on its use.
Moreover, with respect to the liquid crystal displays, inherent problems can be indicated.
Initially, because of its narrow angle of field, the liquid crystal display is not suited for a large-sized display. Additionally, the liquid crystal display is one wherein display is realized owing to the oriented state of liquid crystal molecules, with the problem that the contrast may change depending on the angle even within a range of its angle of field.
On the other hand, in view of a drive system, the active matrix system of a liquid crystal display exhibits a response speed sufficient to deal with a motion picture. Nevertheless, since a TFT drive circuit is used, it is difficult to scale up a picture size by the influence of pixel defects. In addition, the active matrix system is not beneficial from the standpoint of cost saving.
The simple matrix system, which is another type of drive system, is low in cost unlike the above case and is relatively easy in scaling-up of a picture size, but has the problem that its response speed is not sufficient to cope with a motion picture.
In contrast to such liquid crystal devices as set out above, plasma display devices, inorganic electroluminescent devices, organic electroluminescent devices and the like, which are described hereinbelow, are those devices belonging to the spontaneous emission type.
The plasma display device makes use, for display, of plasma emission in a low pressure gas, and is suited for scaling-up and realization of large capacitance. However, this device has the problem on thinning and costs. In addition, an AC bias of high voltage is necessary for its drive, and thus, the plasma display device is not suitable for use as a portable device.
As for the inorganic electroluminescent device, a green light emission display has been put on the market, and necessitates several hundreds of voltages for AC bias drive like the plasma display device, thus being not accepted by users.
In fact, as a result of further technical developments, emissions of three primaries of RGB, which are necessary for a color display, are now in success. Because inorganic materials are essential for device arrangement, it is not possible to control an emission wavelength such as by a molecular design. In view of this, it is presumed to involve difficulties for realizing a full color display.
The organic electroluminescent device utilizes electroluminescence based on organic compounds, and this phenomenon was discovered about 30 and several years ago. More particularly, in the first half of 1960's, it was observed that when carriers are injected into an anthracene single crystal capable of intensely emitting fluorescence, a unique light-emitting phenomenon (induced by luminescence) occurs. Since then, although an organic electroluminescent device has been studied over a long time, a technical emphasis has been placed mainly on the carrier injection into an organic material because of its low luminance and monochrome and also of the use of a single crystal, and thus, its study has been within a range of a fundamental stage.
Around the time passing the middle of 1980's, circumstances have changed. In 1987, Tang et al. of Eastman Kodak released an organic thin film electroluminescent device that was epoch-making at that time. This is composed of a built-up structure having an amorphous emission layer and enables low voltage drive and emission of high luminance. With the disclosure of this built-up structure as a start, studies and developments of an organic luminescent device were further encouraged. As a consequence, studies have been extensively made, up to now, on the emission of three primaries of RGB, stability, rise of luminance, built-up structure and manufacturing method.
Besides, novel materials for displays have been successively developed while making use of the molecular design that is possible only for organic materials. Thus, organic electroluminescent devices have appeared in succession as having good features such as of DC low voltage drive, thinning, spontaneous emission property and the like. Its applications to a color display has been extensively studied.
The merits and demerits of the typical display devices have been hereinabove set forth along with the history of developments. Because the invention is directed to improvements in organic and inorganic electroluminescent devices among these display devices, the problems that are now involved in electroluminescent devices and are required, by many fields of industry, to be overcome are described below.
The contrast characteristic is one of main properties of a display. With spontaneous emission-type display devices such as an organic electroluminescent device, an inorganic electroluminescent device and the like, a satisfactory contrast cannot be obtained by the influence of a metal back (i.e., reflection of external light with a metal cathode). Especially, the contrast of an organic electroluminescent device is, at most, as small as approximately 200:1. Thus, there is a demand, from many fields of industries, for the development of a display device having a contrast sufficient to exceed the above-mentioned contrast.
The luminous efficiency of the organic electroluminescent device is still unsatisfactory. Mention is made, as one of the reasons for this, of the fact that an injection efficiency of carriers from an electrode into an organic layer is small.
Further, a great obstacle involved in the development of an organic electroluminescent device is that an anode material is limited almost to ITO, ZnO, SnO2 and related substances, thus placing a great limitation on the energy matching of a hole injection material with the anode material.